1. Field of the Invention
The present invention relates to a system for the passivation of metal surfaces in equipment affected by operating conditions and by agents which promote corrosion. In general, the present invention relates to a system for the passivation of surfaces in chemical plants involving the presence and formation of corrosive compounds, especially where the environmental conditions, such as temperature and pressure, intensify the corrosive action. More particularly, the present invention concerns a system for the passivation of surfaces of metal equipment used in chemical plants, and exposed to the intensive action of highly corrosive compounds, under temperature and pressure conditions higher than ambient conditions.
2. Description of the Prior Art
It is well known that the synthesis of urea is carried out at high temperature, such as on average 180.degree. to 215.degree. C., and high pressure, such as on average 130 to 400 bar. Provided downstream of the synthesis section are a number of decomposition stages of urea synthesis by-products, i.e., by-products which have not been transformed into urea. Such by-products include ammonium carbamate, which through heat is decomposed into NH.sub.3 and CO.sub.2, and is separated from the synthesis elements as gaseous NH.sub.3 and CO.sub.2. The gaseous NH.sub.3 and CO.sub.2 are condensed in successive condensation stages thereby forming aqueous solutions of ammonium carbamate and/or ammonium carbonate, which are recycled to the synthesis section. The urea is meanwhile concentrated in successive stages operating at decreasing pressure until the final vacuum concentration stage is reached from which virtually pure melted urea is obtained, which is then sent to a finishing stage operated with various techniques.
Various systems have been proposed for the passivation of the equipment used in the respective stages mentioned above, which is subjected to corrosion by the corrosive compounds treated in it. For example, Belgian Patent No. 625.397 describes the use of oxygen as a passivating agent at 180.degree. C. and 270 kg/cm.sup.2 for the surfaces in a urea synthesis reactor made of stainless steel containing up to 19% Cr and 14% Ni. In general, oxygen can also be replaced by other passivating agents, such as, for example, hydrogen peroxide and alkali metal peroxides or alkali earth peroxides.
In European Patent 0096151, a passivation system is described for strippers, where the effluent from the urea synthesis reactor is treated, at high temperature and a pressure of between 120 and 240 kg/cm.sup.2, as a thin falling film countercurrent with NH.sub.3 or CO.sub.2. In this method, to a first passivating agent consisting of an oxygen-containing gas, which is introduced from the bottom of at least one stripper, is added as a second passivating agent a liquid injected from the top of the stripper. The liquid is selected from hydrogen peroxide, alkali metal persulphate or perborate, peracetic acid and organic peroxide. Oxygen may be introduced into the plant as pure oxygen, or mixed with air or with hydrogen peroxide.
The oxygen in gas form is introduced into the system by injecting it into the CO.sub.2 before it is compressed, or into the ammonia entering the synthesis zone, or as hydrogen peroxide into the various liquid flows upstream of the entrance to the equipment to be protected.
The passivating system using hydrogen peroxide requires, in any case, the simultaneous injection of gaseous oxygen, either as air or as pure oxygen as mentioned before.
The above-mentioned passivation systems are used to protect metal materials usually employed in industrial plants for the production of urea, such as various types of stainless steel, titanium, etc.
Besides the above-mentioned passivating agents, others have been put forward in, for example, DE-A-1800755, such as soluble ammonium nitrite, soluble sodium nitrite, and other substances not used industrially.
Nowadays the technique universally adopted to passivate metal surfaces in contact with the solutions and vapors present in the plant's various stages is to send to the synthesis reactor air and oxygen by injecting them into the CO.sub.2. In some cases, besides this injection, hydrogen peroxide is also introduced into the liquid flow upstream of the equipment to be passivated.
The oxygen content injected into the CO.sub.2 has a concentration of between 0.2% and 0.8% in volume and this causes some problem, of which just a few are pointed out as follows:
1. The oxygen injected as air enriches the reactor with nitrogen, with the obvious consequence that the synthesis zone is enriched with the inert gas. This creates a gas phase, the consequence of which is a reduction in urea yield in the reactor.
2. Since the CO.sub.2 comes from the synthesis gas decarbonation section for the production of ammonia, it contains H.sub.2, N.sub.2, CO and CH.sub.4 in such proportions that together with the oxygen they create explosive mixtures.
It has been found that to avoid the possibility of an explosion, the amount of oxygen injected into the CO.sub.2 must be below 0.2% vol. However, under these conditions it has also been observed that the protective action over the metal surfaces is greatly reduced thus leaving them exposed to the corrosive action of both liquid and gas substances present in the various parts of the equipment.